The cyw43-driver now provides the Bluetooth initialisation code, making
`drivers/cyw43/cywbt.c` obsolete. To use the new code a port must enable
the `CYW43_ENABLE_BLUETOOTH_OVER_UART` option.
Some ports have yet to migrate to the new code, so in the meantime they can
explicitly add the old source to their source list and continue to use it
without change.
Signed-off-by: Damien George <damien@micropython.org>